There are different occasions in which it is desirable to operate an engine gas exchange valve in one of two modes, the valve, in one mode, having a single main event and, in the other mode, having both a main and an auxiliary event.
For example, an exhaust valve is conventionally opened during the exhaust stroke and closed near top centre at which time the intake valve is opened. Overlap between the opening events of the two valves results in internal exhaust gas recirculation which improves engine performance under some engine operating conditions but not others. To optimise the degree of internal EGR under different operating conditions, it is desirable to have an auxiliary opening of the exhaust valve during the intake that may be selectively enabled and disabled. A similar effect can be achieved by providing an intake valve with a selectable event during the exhaust stroke.
As a further example, opening an exhaust valve near top dead centre on the firing stroke acts as a compression brake which is desirable under some conditions but not others. A system that allows an auxiliary event to be selectively enabled and disabled allows an engine compression brake to be activated as required.